1. Field of Invention
This invention relates to the field of sensing aircraft and other objects and is part of the See and Avoid (SAA) function for manned aircraft and the Detect, Sense and Avoid (DSA) function for remotely piloted vehicles (RPVs) and unmanned aerial vehicles (UAVs). RPV is an older term for UAV. Drone is another older term for UAV. “UCAV” shall mean “Unmanned Combat Aerial Vehicle.” UCAV is also sometimes defined as an “Uninhabited Combat Aerial Vehicle.” UCAV is a UAV that is intended for use in combat. UAS means “Unmanned Aerial System.” UCAS means “Unmanned Combat Air System.” The characteristics all these vehicles have in common is that there is no human pilot onboard, and although they may be operated autonomously they can also be controlled by a remotely located operator or pilot. The term UAV shall be used as a generic term for such vehicles. Detect, Sense, and Avoid (DSA) is also commonly called Sense and Avoid (SAA) since “Detect” and “Sense” mostly mean the same thing. This invention is directed to the “See” in “See and Avoid” and the “Sense” in “Sense and Avoid.” Automatic Dependent Surveillance-Broadcast (ADS-B) is the system by which an aircraft periodically transmits a message comprising its identification, location, altitude, and heading. The term “datastream” means the stream of data comprising the ADS-B message. The term “bitstream” means the same as “datastream.”
2. Prior Art
In an aircraft with the pilot onboard, Sense and Avoid is called See and Avoid. FAA Regulations do not give much guidance for seeing other aircraft.    Right-of-way rules: Except water operations 14 CFR §91.113(b) [IDS Cite 2]:            (b) General. When weather conditions permit, regardless of whether an operation is conducted under instrument flight rules or visual flight rules, vigilance shall be maintained by each person operating an aircraft so as to see and avoid other aircraft. When a rule of this section gives another aircraft the right-of-way, the pilot shall give way to that aircraft and may not pass over, under, or ahead of it unless well clear.            Right-of-way rules: Water operations 14 CFR §91.115(a) [IDS Cite 3]            (a) General. Each person operating an aircraft on the water shall, insofar as possible, keep clear of all vessels and avoid impeding their navigation, and shall give way to any vessel or other aircraft that is given the right-of-way by any rule of this section.When operating under Visual Flight Rules the idea is to look out small windows providing a limited field of view and hope you see any nearby aircraft in time to avoid a collision. This is made more difficult because of the wide range of aircraft sizes and speeds. (Is it a large aircraft far away or a small aircraft much closer?) This is even more difficult under instrument flight rules where there may be no visibility.        
UAVs have special problems sensing other aircraft.                1. If the UAV is flown manually by a remote pilot looking at the video produced by a camera mounted in the nose of the aircraft the field of view will be too limited to see other aircraft other than those directly ahead.        2. If the UAV is flown autonomously there is no human pilot. If the flight is supervised by a human operator the problem remains that the field of view from a camera mounted in the nose of the aircraft will be too limited.        
A system by which an aircraft periodically transmits its identification, location, altitude, and heading is taught by U.S. Pat. No. 5,153,836 Universal dynamic navigation, surveillance, emergency location, and collision avoidance system and method issued Oct. 10, 1992 to Fraughton et al. [IDS Cite 4] and was materially adopted by the FAA as Automatic Dependent Surveillance-Broadcast (ADS-B). According to the article Gulf of Mexico Helo Ops Ready for ADS-B in Aviation Week & Space Technology (Feb. 26, 2007, page 56) [IDS Cite 5]:                By the end of 2010, FAA expects to have the ADS-B system tested and operationally acceptable for the NAS, with Houston Center providing services in the Gulf region. By 2013, all of the U.S. is scheduled to be covered with ground infrastructure.        
The ADS-B system is used as a radar in U.S. Pat. No. 7,414,567 ADS-B radar system issued Aug. 19, 2008 to Zhang et al. [IDS Cite 6] Zhang modifies the standard ADS-B equipment by providing for random phase modulation that is added to a standard ADS-B waveform utilizing it as a primary radar signal. Phase coherent radio-frequency electronics are used for modulations and de-modulations and the phase modulator can be inserted bit by bit at a 180 degree phase shift per bit change. (See Column 3, lines 49-64)
One of Zhan's reasons for introducing random phase modulation is to raise the transmit spectrum sidelobes. (See Column 8, lines 23-27).
The reason why raising the transmit spectrum sidelobes is desirable appears to be because (1) raising the peaks in the transmit spectrum will increase the peaks in the spectrum of a reflected target signal and (2) the peaks in the spectrum of the reflected signal are used to compute the angles-of-arrival of the reflected target signal, i.e. the target. (See Column 7, lines 10-26)
Note that determining the angles-of-arrival of a signal cannot be done with a single omnidirectional antenna. It requires more than one antenna. Indeed, Zahn uses two antenna arrays, i.e two arrays of antennas. See Column 6, lines 39-45:                Referring again to FIG. 1, antenna system 50 includes a plurality of antennas, which, in the embodiment shown, include a top circular array antenna 51, and a bottom circular array antenna 52. The arrays include individual elements 53 (FIG. 4). Circular array antennas 51 and 52 may be of a 4- or 8-element type as is common for TCAS systems or a 16-element type antenna.Another reason for introducing random phase modulation is for identifying the reflected signal. See Column 6, lines 26-36:        The phase shift is inserted pulse-by-pulse, and the pulse-position modulation with phase shifting is synthesized digitally at the I/Q baseband and up-converted to 1090 MHz carrier frequency. In the embodiment shown, a 180.degree. phase shift is added pulse-by-pulse in a random manner. In addition, the random phase shift code is put in memory during each message transmission. When reflected pulses are received, the system will try to match the amplitude and phase changes from pulse to pulse in a pulse-compression or matched filtering process, as discussed below in further detail.There is a weakness in Zahn's system when the target is an Adversary. The Adversary can receive Zahn's signal, see that the phase of the bits is being changed, and know that Zahn is using his ADS-B Radar. The Adversary will have to assume he has been detected and has lost the element of surprise. As a result, Zahn has also lost an element of surprise because there is value in detecting an Adversary who does not know he has been detected. The invention of the current inventor teaches such a system.        
In European Patent Application EP2136222 Validity check of vehicle position information published Dec. 23, 2009 (Persson, et al.) [IDS Cite 7] ADS-B is not used as a radar. Instead, Persson assumes that the target is broadcasting ADS-B signals. The direction to the target is determined using a directional antenna. The range to the target is determined when the target is sending a proper time-synchronized ADS-B signal. If the target is not broadcasting a valid ADS-B signal then radar must be used. See page 3, paragraphs 19, 20, and 21.
U.S. Patent Application Publication Number 20110140950 Validity check of vehicle position information transmitted over a time-synchronized data link published Jun. 16, 2011 [IDS Cite 8] contains the same disclosure as the above European Patent Application EP2136222. The title makes it clear that the invention requires a time-synchronized data link. Note that the U.S. Patent Application Publication lists only Svante Anderson as the inventor. Svante Anderson is listed as a co-inventor on the European Patent Application.
Where ADS-B is relied upon to prevent mid-air collisions, an aircraft that does not have the equipment installed (or ADS-B is broken or has been deliberately turned off) is a hazard to itself and other aircraft in the vicinity.
Current Practice in Flying UAVs
The current practice in flying UAVs in civilian airspace is typified by the report Sensing Requirements for Unmanned Air Vehicles by AFRL's Air Vehicles Directorate, Control Sciences Division, Systems Development Branch, Wright-Patterson AFB OH, June 2004, which relies on computer-intelligence to use sensors to sense and avoid other aircraft. [IDS Cite 9]
According to the presentation entitled Developing Sense & Avoid Requirements for Meeting an Equivalent Level of Safety given by Russ Wolfe, Technology IPT Lead, Access 5 Project at UVS Tech 2006 this had not changed as of Jan. 18, 2006. [IDS Cite 10] Access 5 was a national project sponsored by NASA and Industry with participation by the FAA and DOD to introduce high altitude long endurance (HALE) remotely operated aircraft (ROA) to routine flights in the National Airspace System (NAS). Access 5 started in May 2004 but when NASA withdrew its support (and funding) the Industry members decided not to spend their own money and Access 5 was dissolved at the end of 2005.
The presentation Integration into the National Airspace System (NAS) given by John Timmerman of the FAA's Air Traffic Organization (Jul. 12, 2005) essentially says that under current UAS Operations in the NAS UAVs should not harm other aircraft or the public. (Page 3: “While ensuring ‘no harm’ to other NAS customers and public”) [IDS Cite 11]
The article Zone Ready for Drone, Apr. 7, 2006, on the web site for the FAA's Air Traffic Organization Employees states that [IDS Cite 12],                Since March 29, a temporary flight restriction . . . has limited access to the airspace along almost 350 miles of the border, expanding an earlier TFR near Nogales. The restriction is in effect nightly from 6 p.m. to 9 a.m., although that time can be expanded by issuance of a Notice to Airmen. Aircraft wishing to fly in the TFR when it is active must receive authorization from air traffic control prior to entry. Once in, pilots are required to maintain two-way communication with ATC and transmit a discrete transponder code.The reason for the TFR is to enable Predator UAVs to patrol the border. The article quotes Stephen Glowacki, a Systems Safety and Procedures specialist with the FAA's Air Traffic Organization as saying:        This is an extreme situation that has been presented to us,” states Stephen Glowacki, a Systems Safety and Procedures specialist with the FAA's Air Traffic Organization, stressing the nation's security. “We have been working with U.S. Customs and Border Protection to try and answer this situation.”        Inserting UASs into the National Airspace System is not a simple feat. According to Glowacki, the technology and certification that will permit unmanned aircraft to “see and avoid” other air traffic is still eight to ten years away. In the mean time, a carefully controlled environment is needed.        
From Quadrennial Roles and Missions Review Report, Department of Defense, January 2009, page 29 [IDS Cite 13]:                U.S. Joint Forces Command Joint UAS Center of Excellence has identified three areas necessary to ensure access to applicable classes of the National Airspace System: (1) Airworthiness Certification; (2) establishment of standardized basic UAS qualifications consistent with Federal Aviation Administration guidelines for each class of airspace; and (3) development of sense and avoid technology. Working with the Services, the U.S. Joint Forces Command Joint UAS Center of Excellence will ensure these areas are addressed during UAS development.(Emphasis added.)        